The present invention concerns the preparation of chlorinated pyridine derivatives and particularly concerns the preparation of 2,3,5,6-tetrachloropyridine and 2,3,4,5,6-pentachloropyridine. Chloro-substituted derivatives of 2-chloro-6-(trichloromethyl)pyridine are also prepared according to the present invention.
The chlorinated pyridine derivatives of the present invention are known compounds having been previously prepared by a number of processes. These compounds have uses such as herbicides, pesticides, etc., and are also employed as chemical intermediates in the preparation of other highly desired herbicide or pesticide products. Previous methods for preparing such compounds include those described in U.S. Pat. Nos. 3,538,100 and 3,186,994 and the prior art noted therein. Thus, according to the U.S. Pat. No. 3,538,100 pentachloropyridine and 2,3,5,6-tetrachloropyridine (hereinafter referred to for convenience as "Penta" and "Tetra" products, respectively,) have been prepared by chlorination of liquid 2,6-dichloropyridine at temperatures of at least about 180.degree. C. and in the presence of a metallic halide catalyst. Polychloropyridines, including Penta and Tetra products, are also produced according to the U.S. Pat. No. 3,186,994 by chlorinating in the absence of a catalyst, a polychloro-(trichloromethyl)pyridine reactant in the liquid state at a temperature of at least 160.degree. C., preferably under irradiation with ultraviolet light.